Oil lubricated roller bearings are subjected to both radial and axial pressures. Radial pressure causes rolling friction, whereas axial pressure causes sliding friction, which is more dangerous. The lubrication between the guide surfaces of the bearing rings and the end surfaces of the rolling bodies is a difficult problem in such bearings. Problems arise because the conditions for maintaining a lubricant film capable of supporting loads at these points are unfavorable at both low rotational speed and particularly when rotation begins. When the bearing is at rest, the lubricant flows out of the gap between the hollow bodies and moving surfaces, causing the lubricant film thickness to be reduced to zero. With high axial loads, moving surfaces become heated and damage to the parts in contact can occur.
In order to reduce the dangerous sliding friction, it is known in the art to introduce additional sliding means, specifically, ball bearings between the end surfaces of the rolling bodies and the guide surfaces of the bearing rings. The assembly of the balls in the bearing is, however, difficult. Thus, as disclosed in German Patent Specification No. 102,583, one or more freely floating rings have also been employed as additional sliding means between the rolling bodies and the guide surface. However, such a procedure also requires a high expenditure in material and work.
The aim underlying the present invention essentially resides in providing a method for increasing endurance and reducing the friction of oil lubricated conical roller bearings that is simple to carry out and inexpensive.
In accordance with the aim of the present invention, the life of the bearing can be substantially increased and friction greatly reduced by coating the guide surface with a lubricating paint. Advantageously, the paint is insoluble in oil and contains a solid lubricant, in particular, molybdenum disulphide.
The use of molybdenum disulphide as a lubricant in rolling bearings is known, as in, for example, German Patent Specification No. 976,124. However, this involves dry rolling bearings in which molybdenum disulphide represents the only lubricant. Thus, according to German Patent Specification No. 976,124, the bearing is first etched with a rough surface. A suspension of molybdenum disulphide in a slightly volatile solvent is introduced into the pores of the bearing. German Patent Specification No. 976,124 also describes a method where a layer of molybdenum is applied to the metal surface of the bearing. This layer is then converted to molybdenum disulphide by means of a chemical process on the surface of the bearing. Disadvantageously, this method is extremely expensive and somewhat difficult to control.
It is also known to lubricate rolling bearings by applying a lubricating paint to the running grooves of the races, as in, for example, German Offenlegunsschrift No. 21 20 917. Disadvantageously, however, the lubrication of the bearing by lubricating paint is of a relatively short duration. Another disadvantage is that larger bearing clearance must be available between the rolling bodies and the running groove compared with oil lubricated bearings. This has further disadvantageous effects on endurance, and, due to the additional dynamic forces, also on the running and vibration bahavior.